1. Field of the Invention
This invention relates to an image sensor and a method for producing the same, in which the light receiving area of the sensor is restricted by a light shielding layer or the like.
2. Description of the Prior Art
A contact type image sensor having a length substantially equal to the width of the original, and utilizing an amorphous semiconductor such as amorphous silicon (a-Si), or a polycrystalline thin layer such as cadmium sulfide (Cds)--cadmium selenide (CdSe) and the like as a photoelectric conduction layer has been widely known, and the application thereof to the field of an image reading apparatus of a wide reading area is attracting attention because it requires no image-reducing optical system. A fundamental construction of an image sensor formed into a sandwich type is shown in FIG. 19 wherein a photoconductive layer 4 is interposed between a lower electrode 2 formed on a substrate 1 and an upper electrode 3 made of a transparent substance. In the contact type image sensor, a plurality of sensor elements (for instance, in a case of 8 dots/mm, 1728 elements for the fourth size of A series, and 2048 elements for the fourth size of B series of Japanese Industrial Standard) are arranged in parallel on a long substrate. In order to assure correct reading, the sensor elements must be completely independent from each other, and the light-receiving portions of the sensor elements must have equal areas with respect to each other. Especially, the equal area requirement must be strictly observed so that the areas of the light-receiving portions affect greatly on the characteristics of the sensor.
Various procedures have been worked out for equalizing the areas of the light-receiving portions. For instance, in a most basic construction of the contact type sensor, the lower electrode 2 and the photoelectric conductive layer 4 are both divided in accordance with the sensor elements as shown in FIG. 20, while the transparent upper element 3 is also divided in a principal portion thereof, so that a region wherein the photoelectric conductive layer 4 is placed between the thus divided parts of the upper electrode 3 and the lower electrode 2 is defined to be the light-receiving portion of each sensor element.
With this construction, however, the production method of the sensor is complicated because the photoelectric conductive layer and upper and lower electrodes must be produced by photolithoetching processes, respectively. Furthermore, because of the deviation of patterns and the like, the light receiving surface areas of the sensor elements tend to be varied, and, during the photolithoetching processes and the like, the end portions of the photoelectric conductive layer corresponding to the boundary portions of the masks tend to be contaminated or damaged, thereby reducing the reliability and the yield of the production of the contact type sensor.
In addition, in the case of a contact type image sensor which utilizes non-doped amorphous silicon as the photoelectric conductive layer, the isolation between adjacent bits (or adjacent sensor elements) is frequently omitted in view of a high resistance (10.sup.9 .OMEGA.cm at dark time) of the amorphous silicon. In this case, the photoelectric conductive layer 4 and the upper electrode 3 are respectively formed integrally, and only the lower electrode 2 is divided into separate pieces as shown in FIG. 21. With this construction also, the light receiving areas are defined by the overlapped portions of the upper electrode and the lower electrode portions thus divided. In this case, the photolithoetching process is utilized only for the formation of the lower electrodes, and the upper electrode is selectively deposited by a sputtering method and the like utilizing a metal mask or else, thereby simplifying the production process of the contact type sensor.
In this case, however, the upper electrode must be produced by placing the metal mask in close contact with the photoelectric conductive layer mode of a clean amorphous semiconductive substance for realizing a high precision pattern. The direct contact of the metal mask with the amorphous semiconductor layer tends to cause dust deposition or surface damage of the semiconductor layer, and furthermore charged particles tend to collect during the sputtering operation on a surface portion T corresponding to the edge of the metal mask, thus causing variation in the surface composition of the photoelectric conductive layer contacting the metal mask. Such variation of the surface composition tends to cause short-circuit between the sensor elements and reduces the productivity of the same.
Even in a case where the light receiving areas of the sensor elements are made equal, there still remains another problem that the outputs of the sensor elements are differentiated by the difference in the electrostatic capacities of the sensor elements.
The constructions of a sensor portion and a connecting portion of the image sensor is shown in FIG. 22(a) and also in FIG. 22(b) which is a sectional view taken along the line a-b in FIG. 22(a). By the plurality of lower electrodes 102 arranged in a row on a substrate 101, an upper transparent electrode 103, and a photoelectric conductive layer 104 interposed between the lower and upper electrodes 102 and 103, a plurality of light receiving elements 105 are formed, each of which is indicated in FIG. 23 in the form of an equivalent circuit diagram as a parallel connection of a photodiode 105a and a capacitor 105b. In the shown contact type image sensor, the light receiving elements 105 of a required number are arranged on a long substrate in the main scanning direction of the sensor at a density (of for instance 8 elements/mm) which is required for assuring a desired resolution of the original, and the light receiving elements 105 are connected through a connecting portion 106 to a driving portion D of the contact type sensor. The driving portion D comprises a power source 107, a shift register 108 and a plurality of MOS type field effect transistors 109 (MOSFET) which perform switching operations under the control of the shift register 108 for connecting the power source 107 to the sensor elements 105 successively. When each MOSFET 109 is successively ON-OFF operated for one sequence of operation under the control of the shift register 108, a closed loop is successively formed between the power source 107 and respective sensor element 105, so that an electric charge is stored in the stray capacitance 105b of the sensor element 105 and a stray capacitance 106b of the connecting portion 106.
The electric charge is neutralized in accordance with the amount of incident light applied to the sensor element, and when the stray capacitances 105b and 106b are again charged by the ON-OFF operation of the MOSFETs under the second sequence of control operation of the shift register 108, a current corresponding to the not-neutralized electric charge stored in the capacitances 105b and 106b of each sensor element flows through an output signal line 110. The electric current flowing through the line 110 is read out as an output signal of the contact type image sensor corresponding to each bit. The above described operation is repeated for each scanning line until the original is read out completely.
The connecting portion is ordinarily provided on the substrate 101 for connecting the sensor elements to the driving portion D. In this case, it is inevitable to cause difference in length of the connections between the shift register or MOSFETs and the sensor elements depending on the positions of the sensor elements, and hence deviations between the values Cx of the stray capacitances 106b formed along the connections of the portion 106 depending on the locations of the sensor elements.
When the circuit of one of the light receiving elements shown in FIG. 23 is analyzed by use of Laplacean operator S and when it is assumed that the value of the stray capacitance 105b is Cs, the initial charge of the capacitance 105b is V(a) (V.sub.(0) =V.sub.B), and the photoelectric current flowing through the photodiode 105a is represented by Ip, an equivalent circuit as shown in FIG. 24(a) is obtained. This circuit is further simulated by a circuit shown in FIG. 24(b) when no light is applied to the light receiving element, and also by a circuit shown in FIG. 24(c) when light is applied to the light receiving element.
Thus from FIG. 24(b), ##EQU1## and from FIG. 24(c), ##EQU2## On the other hand, ##EQU3## from Equation (1), ##EQU4## from Equation (2), ##EQU5## Thus from the Equations (3), (4) and (5), ##EQU6## and ##EQU7##
From Equation (6), it is apparent that the output read out by each bit of the contact type image sensor shown in FIG. 23 is varied by the capacitances 105b and 106b.
In a case where the sensor elements are formed to be equal to each other, the values Cs of the stray capacitances 105b are all held to be constant, and therefore the output from each bit of the sensor element is varied by the value Cx of the stray capacitance 106b formed along the connection line 106, and the difficulty of the output of the contact type image sensor being deviated by the capacitance 106b cannot be eliminated.